Volume control



March 27, 1934. E. E. UNGER 1,952,736 vowus CONTROL I Filed July "27,1931 2 Sheets-Sheet 1 a Q 7 Z March 27, 1934. E. E. UNGER VOLUME CONTROLFiled July 27, 1931 2 Sheeiis-Sheet 2 Patented Mar. 27, 1934 UNITEDSTATES PAT VOLUME CONTROL Illinois Application July 27, 1931, Serial No.553,248

2 Claims.

This invention relates to radio receiving means in general and moreparticularly to meansfor controlling the volume of reception of radiobroadcast programs and other radio communications. In its broader aspectthis invention is concerned with the control of the volume output ofthermionic tubes, especially in a communication circuit, regardless ofthe function that the particular tubes have in the circuit.

It has been heretofore proposed to control the volume output of radioreceiving sets by controlling the grid bias on the amplifying tube ortubes. There are conditions under which this is unsatisfactory becausesuch a high value of grid bias may be necessary as to cause theamplifier to function as a detector, and thus considerably distort thesignal. To reduce this difiiculty it has been proposed to combine withthe grid biasing means a variable shunt for the input side of the tube,to shunt out a portion of the received signal. This takes the form of apotentiometer with one end connected to the grid bias lead, the otherend connected to the antenna lead, and the rider, or movable armgrounded. In using the potentiometer type volume control I have foundthat as the grounded movable arm is shifted along the resistance to varythe grid bias it will also vary the antenna to ground resistance andthis variation, together with variation in contact resistance as thecontact arm slides along, will produce a variation on the input side ofthe tube. This is amplified through the set and ultimately appears asnoises emanating from the loud speaker. This is commonly known as noisyvolume control. It is an object of the present invention to correct thisdefect in potentiometer type volume controls. I

I accomplish this end by fixing the potential of one point on thepotentiometer. The potential at which this point is fixed is preferablyground potential, although it is within the purview of the presentinvention to fix this potential at any other desirable value. By thisarrangement, as the slider moves toward the ground point to vary thegrid bias it will produce no variations in the input circuit to thetube. As the slider moves past the ground point towards the antenna itwill not affect the grid bias.

It is a further object of the present invention to provide an improvedpotentiometer suitable for use in circuits as above set forth. Thepotentiometer comprises a variable resistor, which may be varied in anypreferred manner, said resistor having a tap intermediate its ends,

which tap is grounded directly or through a fixed resistor.

The attainment of the above and further objects of he'present inventionwill be apparent from the following specification, taken in conjunctionwith the accompanying drawings forming a part thereof.

In the drawings:

Figure 1 is a fragmentary circuit diagram illustrating the principles ofmy invention;

Figures 2 and 3 are similar circuits, illustrating modified forms of myinvention;

Figure 4 is a plan view of one .form of potentiometer suitable for usein the circuit shown in Figure 3;

Figure 5 is a view similar to Figure 4 with the contact making springremoved;

Figure 6 is a fragmentary sectional view taken along the line 6-6 ofFigure 4;

Figure 7 is a fragmentary circuit arrangement illustrating a modifiedform of my inven-- tion wherein the antenna signals are attenuatedbefore the grid bias is increased;

Figure 8 is a fragmentary circuit diagram illustrating the applicationof myvolume control to the plate circuit of an intermediate tube of aradio set; V V

Figure 9 is a fragmentary circuit diagram illustrating the applicationof my invention to the grid input side of an intermediate tube of aradio set; and i Figure 10 is a fragmentary circuit diagram illustratingmy invention as applied to the coupling between the plate and the gridof two adjacent tubes of a radio set.

Reference may be had now more particularly to Figure l of the drawings,wherein I have shown one tube of a typical amplifying circuit to whichmy invention has been applied. An antenna or pick-up device 1 is coupledto a thermionic tube 2 by means of a coupling 3. The coupling 3 may beof any preferred type insofar as concerns the present invention and Ihave herein shown, by way of illustration, a well known couplingcomprising an air core transformer whose primary 5 is connected betweenground and the antenna, and whose secondary is in multiple with avariable condenser 6 and connected between ground and the grid '7 of thetube 2. The tube 2 may be of any preferred type and includes a cathode 8and an anode 9 in addition to the grid 7. The cathode may be heated inany preferred manner. As herein illustrated the tube 2 is of the heatertype wherein the cathode is raised to proper temperature by a suitableheater, not shown.

A comparatively high resistor 10 is connected between the antenna orinput circuit 1 and the cathode 8, said resistor being provided with amovable grounded contact 11. The resistor is provided wth a grounded tap12 intermediate its ends, for a purpose to be more fully set forth asthis description proceeds. A resistor 13 is provided for preventing thegrounding of the cathode by the contact 11.

I have not illustrated the output circuit of the tube 2, including theanode voltage supply, since this may be of any preferred type. It issufiicient here to state that the circuit from the anode 9 extendsthrough the usual arrangements back to the cathode 8 by Way of ground,whereby the circuit is completed through the grounded contact 11 or theground tap 12 and the portion of the resistor 10 to the left thereof. Apotential gradient'is thereby set up across the resistor with the resultthat the cathode is maintained positive with respect to ground. Sincethe grid 7 of the tube is connected to ground the cathode of the tube istherefore positive with respect to the grid, or the grid is negativewith respect to the cathode. This is the proper condition for efiicientoperation of the tube as an amplifier. It is well known that theamplification of the tube varies inversely as the negative grid biasingvoltage varies. Therefore, as the contact 11 is moved in a counterclockwise direction from the position shown, the grid bias is increasedand the amplification of the tube is decreased, thereby decreasing thevolume output of the tube.

As is Well known, the variation in grid bias can not, alone, be used toproduce the full volume control, since, to get a very great reduction involume it might be necessary to have such a high grid bias that the tube2 will cease acting as an amplifier. For this reason the antenna isconnected to the potentiometer so as to provide a variable shunt for aportion of the received signal that would otherwise flow through theprimary of the transformer 5. As the contact 11 moves towards itsextreme position in a counter clockwise direction it gradually decreasesthe amount of resistance shunting the primary of the transformer 5 andthereby reduces the current flow through the transformer, thus varyingthe volume output of the tube.

I have found that the provision of the grounded tap 12 on thepotentiometer resistance 10 alters the mode of operation of the volumecontrol in a decidedly advantageous manner and produces a new and highlydesirable mode of operation in a circuit such as has been described, andit is with this feature that the present invention is primarilyconcerned.

Consider first the mode of operation of the volume control in theabsence of the grounded tap. Assume that the contact is at its extremeright hand position and is being moved in a counter clockwise directionto decrease the volume output of the tube. At the commencement of themovement the grid biasing resistance is low and the resistance shuntingthe primary of the transformer is very high. Therefore, the initialmovements of the contact 11 will produce an appreciable variation in thegrid biasing resistance and only a very small proportionate change inthe resistance shunting the primary of the transformer 5. This willcontrol the volume prmarly by control of the grid bias. As the contact11 arrives at positions further advanced along the resistor 10,additional movement will not only vary the grid bias but will also vary,to an appreciable extent, the resistance shunting the input to the tube2. This introduces two rather difiicult problems. First, it is quitedifiicult to produce a resistor of the above type which will affordstraight line attenuation of the volume, or anything even approachingstraight line attenuation. Secondly, it is very difficult to produce avolume control of this type which will not be noisy. Both of the abovedifficulties arise from the fact that variation of the grid bias alsoproduces a variation in the resistance shunting the input circuit to thetube. This last mentioned variation in current flow is amplified throughthe set and finally appears as noise in the loud speaker, or output sideof the set.

The provision of the grounding tap 12 removes the above mentioneddifficulties. In this case, as the grid bias is varied by movement ofthe contact'there is no appreciable corresponding variation in theresistance shunting the input side of the tube 2, therefore the noiseswhich were heretofore produced on this account are eliminated. When thecontact 11 moves in a counter clockwise direction past the ground point12 to decrease the resistance shunting the input side of the tube 1,there will be no corresponding variation in the grid bias. Thepotentiometer resister 10 is not of uniform resistance throughout itslength. The resistance is correlated to the circuit in which it is usedso that equal increments of length at different points on the resistorwill produce proportionately equal variations in the volume output ofthe set. The problem of suitably tapering the resistance to correlatethe same to the circuit within which it is to be used is appreciablysimplified by the presence of the grounding tap 12 since by thisarrangement the portions of the resistor on either side of the tap maybe considered as separate units, each independent of the other. Onecircuit may be tapered considering only the results produced by varyingthe grid biasing resistance, and the other portion tapered only to takecare of incoming signal attenuation. In the absence of the grounding tapindependent consideration of these two factors is not possible.

Reference may now be had to the circuit diagram of Fig. 2, illustratinga modified form of my invention. The volume control shown in this figurediffers from that previously described only in that the potentiometerresistance is provided with a gap adjacent the tap 12. This means thatas long as the contact 11 is to the right of the tap 12 the resistanceshunting the input side of the tube 2 will be infinite, or, stated inother words, no part of the signal will be shunted to ground. Theresistance of the resistor portion 21 is made rather high, so as toavoid an abrupt change in volume as the contact 11 first moves past thegap 20 in its counter clockwise motion. This form of volume control isvery efficient in controlling the amplification of initially weaksignals, since no part of the original signal is lost when highamplification is desired, as when the contact 11 is in its extreme righthand position.

In Fig. 3 I have shown a circuit diagram similar to that of Fig. 1,differing therefrom only in that the tap 12 on the potentiometer isconnected to ground through a fixed resistance 25. The resistor 25 isvtherefore always in shunt with that portion of the potentiometerresistor that lies between the tap 12 and the contact 11. When thecontact is in either of its extreme positions the iiiO value of thepoteniometer resistance in shunt with the resistor 25 is so high that itis of ne li i ble effect, and therefore the resistor 25 is thecontrolling resistor. As the movable contact approaches the tap 12 theeffect of the resistor25 diminishes, this resistor being of zero effectwhen the movable contact is at the tap 12. As the movable contact movesfrom one side to the other of the tap 12 the resistor 25 isconnectedeither in the grid biasing circuit or in the antenna pick upcircuit depending upon the position or the contact 11. The resistance 25is of a rather low value whereas the potentiometer resistance is of arather high value. When the contact 11 is moved counterclockwise fromits extreme right hand position to increase the grid bias there is noeffect upon the antenna to ground resistance due to the fact that thelow resistance 25 is shunting the varying portion of the potentiometerresistor between the tap 12 and the contact 11. Therefore there will beno rheostat noises introduced into the input circuit at this time, whenthe amplification or" the tube is the maximum. As the contact 11approaches the tap 12 and the amplification oi the tube 2 has beenreduced, further movement of the contact toward the tap 12 increases thegrid bias and also reduces the antenna to ground resistance. As thecontact moves to the left of the tap 12 it removes the resistance 25from the antenna circuit and includes it in the cathode or grid biascircuit in shunt with the portion of the potentiometer resistancebetween the contact and the tap 12. The efiect of this portion of thepotentiometer resistance in the grid bias circuit gradually decreases asthe contact 11 moves further and further from the tap 12. Thus there isa gradual attenuation of the antenna to ground resistance and of thegrid biasing resistance as the contact moves to one side or the other ofthe tap 12. The resistance 25 however, serves to reduce or substantiallyeliminate the rheostat noises and afiords a ready means for producing aproperly attenuated resistance of the potentiometer. I

Reference may now be had more particularly to Figures 4, 5 and 6 whereinI show a preferred mamier of incorporating the tap 12 and the resistorin a well known type of potentiometer. The potentiometer comprises aframe of suitable insulating material in which is mounted a resistanceelement 31 which may comprise 'a strip of cardboard whose upper surfaceis covered with carbon, graphite, ink, or other desirable conductingmaterial, of varying density throughout the length of the strip so as toprovide a proper resistance attenuation. Terminals 32 and 33 connect theends of the resistance strip 31 with terminal lugs 34 and 35,respectively. A

thin, flexible metal disc 38 is secured at its hub and adapted to beflexed at its rim into engagement with the resistance strip 31. Aswinging arm 40 is secured to the rotatable element or shaft 41 of thepotentiometer and. makes wiping engagement with the rim of the disc 38to press the portion of the rim immediately under the wiper arm 40 intoengagement with the resistance strip. As the arm 40 wipes along thestrip 33 the point of contact between the rim or" the strip 38 and theresistor is shifted thus varying the amount of resistance between eitherof the lugs 34 or 35 and the contact strip. The contact strip 38 isitself held against rotation and is electrically connected through itshub to a terminal 42. This much of the potentiometer herein shown iswell known in the art and does not constitute my invention. Theresistance strip 31 .includes'a short inwardly extending portion 45which is also coated with the resistance material and forming acontinuation of the resistance material on' the strip 31. A contact 46presses against the extension 45 and makes electrical engagementtherewith. The contacting strip 46 is in engagement with the disc 38 andthereby establishes a circuit to the disc 38. The resistance of theportion of the strip 45 between the contact 46 and the portion of thestrip 31 engaged by the contacting disc 38 opposite thereto representsthe amount of fixed resistance represented at 25 in, Figure 3. II" thecontacting portion 46 is brought out further to the rim of the strip 31-there will be a resulting decrease in the value of the hired resistance25. If this contact 46 is brought out to the center of the strlp 31 thevalue of the resistance 25 will be reduced 'to zero therebycorresponding to the system shown in Figure 1. If it is desired toprovide a gap in the potentiometer resistance, as indicated at 20 inFigure 2, this may be accomplished by scraping away the resistancematerial from the cardboard strip 31 along a radim 100 line adjacent theterminal portion 46.

In Figure 7 I have illustrated, in diagrammatic form, an arrangementwherein the antenna signal is attenuatedbefore the grid bias isincreased. The potentiometer comprises two re- 1105 sistance portions 50and 51 over which the contact 11 moves. The resistance 51 isthe'gridbiasing resistance whereas the resistance 50 is the antenna to groundshunting resistance;

While I have herein shown these two resistances 110.

as laid out in a straight line, it is to be understood that they may bearranged in the form of a continuous curve over which the contact 11wipes or swings. There is provided'a grounding switch 55 that isactuated by the contact 11, or 113' by the means which actuates thecontact 11, so that it selectively grounds either the contact 56 or acontact5'7, depending upon the position of the contact 11. When thecontact 11 is to the right of the switch 55 this switch grounds the liqcontact 56. To produce the minimum volume on the output side of t e tube2 the contact 11 is moved to its extreme right hand position. At thistime there is a maximum grid bias due to the fact that the maximumamount of resistance 12 5 51 is included in the cathode circuit. Thisreduces the cathode-anode conductance to its minimum value. At the sametime the switch 55 grounds the contact 56 thereby producing a groundpath for the antenna signal. To in- 30 crease the volume output of thetube 2 the contact 11 is moved to the left as seen in Figure 7. As thiscontact approaches the end of the resistance 51 the gridbias is beingreduced with a resulting increase in the conductance between 35 thecathode and the anode thereby increasing the volume output of the tube2. As the contact 11 moves past the switch 55 to the resistance portion50 it actuates the switch to remove ground from the contact 56 and toplace ground upon the contact 57. Further movement of the contact 11increases the antenna to ground resistance thereby resulting in afurther increase in the output of the tube 2. The resistance 50 is of arather high value so as to afford a rather high 1 5 antenna to groundresistance when the contact 11 is at its. extreme left hand position. Ifdesired the arrangement may be such that the contact 11 moves ofi of theresistance 50 at the end of its travel, thereby entirely removing the antenna ground. When the contact 11 moves to the right past the switch 55it returns this switch to the position shown in the drawings.

The switch 55 is incorporated within the potentiometer structure. Sincethis switch carries a negligible current the entire arrangement may bemade very compact.

Reference may now be made to Figure 8 showing a modified form of myinvention wherein the volume control is located in the plate circuitbetween two adjacent coupled thermionic tubes of a radio receiving set.In this case the potentiometer is indicated at 62 and comprises acontact 63 adapted to move over a resistance comprising the sections 64and 65 A resistance 67 shunts the resistance 65, being connected to thejuncture of the resistances 64 and 65 at the tap 68. The potentiometerresistance is connected at one end to the plate or anode of the tube 60and at its other end is connected to the usual anode voltage supplywhich may comprise a B battery, and B battery eliminator, or itsequivalent.

The contact 63 is coupled to the grid or input side of the tube 61 inany preferred manner. I

have herein shown a standard coupling means comprising an air coretransformer 71 and a tuning condenser 72. It is however to be understoodthat any other desirable coupling arrangement may be provided. Thedegree of amplification of the unit comprising tubes 60 and 61 may bevaried by shifting the contact 63 along the potentiometer resistance,the amplification being maximum when the contact is at the upper portionof the resistance 64 and minimum when the contact is at the lowerportion of the resistance 65. The provision of the resistance 6'? inshunt with the resistance 65 greatly facilitates the proper attenuationof the potentiometer resistance.

In Figure 9 I have shown an arrangement corresponding to the arrangementshown in Figure 8 but difiering therefrom in that here the volume ofcontrol is placed on the input side of the tube 61 rather than on theoutput side of the tube 60 as previously. The tubes 60 and 61 arecoupled by a coupling means which may be of any preferred constructionand has been here illustrated as comprising an air core transformer. Theprimary of the air core transformer is connected between the plate ofthe tube 60and the plate voltage supply. The secondary side of thistransformer is connected across the potentiometer 62, one side of thepotentiometer being grounded, and the movable contact being con.- nectedto the grid of the tube 61. It is apparent that movement of the contact63 will vary the output of the tube 61.

In Figure 10 I have shown the output side of the tube 60 coupled to theinput side of the tube 61 in a well known manner, as by means of an aircore transformer and a variable tuning condenser. The volume iscontrolled by means of the potentiometer 62 connected between the platecircuit and ground, with the movable contact 63 of the potentiometerconnected to the grid of the tube 60 through a small fixed condenser 85.This condenser is provided in order to prevent short circuiting of theplate voltage supply through the primary of the coupling transformer,the potentiometer resistance, the wiper 63, and the secondary of thecoupling transformer to ground. In the absence of the condenser the Bbattery supply might be short circuited when the contact 63 is moved toits extreme left hand position because of the fact that one terminal ofthe B Voltage supply is grounded. This condenser 85 may, if desired, beinterposed between the potentiometer and the connection to the anode orplate of the tube 60.

From the description thus far given it is apparent that my invention maybe employed in a wide variety of forms and is not limited to the preciseembodiments herein shown for illustrative purposes. What I consider newand desire to secure by Letters Patent is:-

I. In a thermionic tube system employing a hot cathode tube having agrid, and wherein the volume output of the tube is controlled both byvarying the grid bias and by variably shunting the grid input circuit, aresistor having one terminal connected to the grid input side of thetube and another terminal connected to the grid bias lead of the tube, agrounded contact movable over the resistor, and a ground connection tothe resistor ata fixed point intermediate the limits of travel of thecontact, said resistor being continuous between the ground connectionand the terminals connected, respectively, to the grid input side or"the tube and to the grid bias lead.

2. Incomblnation with a pick-up device and a thermionic tube having ananode, a cathode and a grid coupled to the pick-up device, means forcontrolling the volume output of the tube, said means comprising agrounded resistor shunting the signals from the pick-up device, agrounded resistor controlling the grid bias, both of said resistorscomprising a single electrically continuous resistor unit with theground point intermediate the ends thereof, whereby the resistor shuntacross the pick-up device is maintained at all times, and a groundedcontact movable over the resistor unit by a continuous motion, first tovary the grid bias without affecting the shunting resistor and then toshift the effective grounding point of the shunting resistor withoutfurther varying the grid bias.

ERWIN E. UNGER.

